1. Field of the Invention
This invention relates to optical fibers and, more particularly, to large-mode-area, multimode optical fibers for high power optical amplifier or laser applications and improved coupling efficiency.
2. Discussion of the Related Art
Because of their high performance and cost effectiveness, rare-earth-doped fiber amplifiers (REDFAs), especially erbium-doped fiber amplifiers (EDFAs), are widely used in silica fiber-optic communication systems such as, for example, long-haul transport and CATV applications. Innovative design and optimization of rare-earth-doped fibers (REDFs), especially erbium-doped fibers (EDFs), have both played a critical role in these applications. In particular, designs that confine the optical mode field and control the erbium distribution enable efficient, low-noise amplification of light at low and medium optical power levels. On the other hand, for high power applications large-mode-area (LMA) fiber lowers the signal intensity, thereby reducing deleterious nonlinear effects, and also increases the pump absorption efficiency. High power REDFAs and rare-earth doped fiber lasers (REDFLs), especially those utilizing ytterbium-doped fibers (YDFs), also have many applications outside the traditional telecommunications industry. For example, high power, LMA, YDFs are used in welding and cutting, laser ranging and target designation, medical applications and pollution detection, and free space communications (e.g., between satellites).
The optical characteristics of a LMA fiber sensitively depend upon the details of its transverse refractive index profile. Conventional wisdom dictates that desirable LMA fibers have a fundamental mode with M2 very near to 1.0, meaning that the optical field of the fundamental transverse mode is very nearly Gaussian in shape because the transverse refractive index profile inside the core is essentially uniform; that is, the refractive index profile is essentially uniform within the transverse cross-section of the core. M2 measures the similarity between the mode field and a true Gaussian function. More specifically, M2=1.0 for a mode having a Gaussian shape, and M2>1.0 for all other mode field shapes. An M2 very near to 1.0 facilitates low loss optical coupling, and furthermore the beam emerging from the fiber may be efficiently collimated or tightly focused to a diffraction limited spot. However, fabricating an LMA fiber with an ideal fundamental mode (M2=1.0) and a uniform core refractive index profile can be difficult due to the tendency of the profile to exhibit a dip in refractive index near the longitudinal axis (also known as a center dip or burnoff). Moreover, LMA fibers with a fundamental transverse mode M2 very near to 1.0 exhibit smaller effective areas and hence lower thresholds for undesirable optical nonlinearities than the fundamental transverse modes of fibers with similar core diameters but pronounced center dips. Finally, when a LMA EDF's core transverse refractive index profile is essentially uniform and the fundamental mode's M2 is very near to 1.0, there is relatively little overlap between the fundamental mode and the outer region of the doped core. As a result, the fundamental mode may experience low amplification efficiency while high-order modes may experience undesirable amplification.
Although the foregoing discussion focuses on LMA REDFs, in many respects it is equally applicable to (i) LMA fibers doped with other gain-producing species, such as chromium and (ii) to LMA fibers that are not doped with any gain-producing species. In the latter case, for example, the LMA fiber might comprise the pigtail of a gain-producing fiber (GPF) or other device, or it might simply be a fiber segment coupling the stages of a multistage optical amplifier.
Thus, a need remains in the art for a LMA fiber with improved optical coupling efficiency.
There is also a need for such a LMA fiber that is suitable for high power optical fiber amplifier and laser applications.